An exemplary field of use of such position regulators can be position control of actuating or control drives. In this case, an electrical nominal value is preset for a control valve. The control valve converts to a working pressure which is applied, for example, to a spring-reset, single-acting pneumatic cylinder. A position sensor can be arranged on its piston rod, to measure the actuating movement in the actuating drive in reaction to the application of a pressure medium, and to feed this back to the control loop. Depending on the electrical nominal-value preset, the control valve connects the working connection of the electropneumatic position regulator to the feed-pressure connection for application to the actuating drive to carry out a movement, or to a vent connection to vent the actuating drive, so as to reset a movement. The actuating drive itself is in turn used to operate a fitting, such as a valve within a pipeline system of a process automation installation for the foodstuffs industry, the pharmaceutical industry, the refinery industry or the like, for example.
The pressure-medium-operated position regulators can assume a defined safety position, in the sense of a so-called “shut-down” in the event of a fault, for example, in the event of failure of the feed pressure, so as to ensure the safety of the fitting, which is operated via the feed pressure, in the process automation installation.
EP 1 758 007 A1 discloses a pressure-medium-operated position regulator of this generic type. The position regulator has a valve mechanism, by means of which a feed-pressure connection, a vent connection and a working connection can be switched variably to produce a working pressure for a downstream actuating drive. For the purpose of presetting the desired switch position, the position regulator has two fluid application surfaces, which are arranged opposite one another and each bound a control chamber. The two control chambers are connected to a common control pressure connection, with the interposition of a restriction device. Each control chamber is connected to a vent opening downstream from the two restriction devices. The control valve device can control the two vent openings, and can also close them at the same time. This symmetrical design with respect to the two fluid application surfaces in conjunction with control via a jointly associated control pressure connection offers the guarantee that the fluidic actuating forces that act on the position regulator when both vent openings are closed at the same time compensate for one another, resulting in a clearly defined position.
For instance, it is possible to preset a basic position of the actuating device when the vent openings are closed, in which the working connection is disconnected both from the feed connection and from the vent connection, that is to say, corresponds to the central, closed switch position of a 3/3-way valve, such that a constant pilot pressure is maintained so as to provide the position regulator with a blocking failure behavior. This blocking failure behavior is achieved here by appropriate electrical control of the position regulator.
In conventional position regulators, the “shut-down” function, which has already been mentioned above, can be used as a safety position for position regulation. In this case, the position regulator switches to safe venting of the position drive, as a result of which the position drive can move the valve mechanism of the position regulator to a safe position, specifically entirely open or entirely closed, with the aid of the spring effect of the integrated reset spring. This “shut-down” function is activated in various fault situations, such as:                a diagnosis unit of the position regulator signals a serious signal-processing fault within the electronic control unit, as a result of which the actuating drive is no longer serviceable, for example “RAM/ROM/NV check not OK” or “Position sensor defective”;        the electrical power supplied to the position regulator is not sufficient to ensure operation; in the case of conventional position regulators, this is the case from less than approximately 3.8 mA or, if the supply voltage is inadequate, such as less than 9.7 V;        control of the “shut-down” function by other events which are determined by, for example, sensors outside the position regulator, and which are passed to the position regulator via a signal input.        
When the position regulator assumes the defined safety position as a result of the events described above or other comparable events, in the case of the conventional pressure regulators, the signaling of the defined safety position takes place only via the electrical connecting line, which generally corresponds to a standardized bus protocol; that is, messages back from the position regulator can also be passed to a superordinate control line. Another known possibility is for a mechanical marking to be applied to the actuating drive which, when viewed from the immediate vicinity, can provide the operator with information that the actuating drive, and therefore the position regulator, has assumed the safety position. In the case of modern man-machine interfaces, a message such as this can also be displayed via an indication text, such as a diagnosis message, for example.
However, the solutions from the conventional art as described above have a disadvantage in that, when the position regulator is viewed from further away, the forms of signaling explained above cannot be observed accurately or at all. Particularly in operating situations in which a plurality of position regulators with actuating drives are arranged alongside one another, even if it is known that one of these position regulators is faulty, it is difficult for the operator to ascertain which of the position regulators is affected by the fault condition.